1. Field of the Invention
The present invention relates to a multivalued FSK demodulation window comparator and, more particularly, to a multivalued FSK demodulation window comparator capable of improving the sensitivity when a reception electric field is weak.
2. Description of the Prior Art
FIG. 1 is a block diagram showing a conventional quaternary FSK demodulation radio section.
A radio signal received by an antenna 1 is amplified by a radio amplifying section 2. The frequency shift data of the resultant FM demodulated signal is converted into an analog voltage value by a quadrature detection circuit in a demodulating section 3. The demodulated analog voltage value is converted into 2-bit digital parallel data consisting of a most significant bit MSB and a least significant bit LSB by an MSB comparator 4 having the input/output characteristics shown in FIG. 2 and an LSB comparator 5 having the input/output characteristics shown in FIG. 3. The MSB is the result obtained by determining the polarity of the frequency shift. The LSB is the result obtained by determining the magnitude of the absolute value of the frequency shift.
FIGS. 4A, 4B, and 4C are graphs showing a demodulated signal and operations of the MSB and LSB comparators in the prior art.
To determine the polarity of a frequency shift, the MSB comparator 4 has a reference level set to a level V.sub.REF corresponding to the center of the waveform. To determine the magnitude of a frequency shift, the LSB comparator 5 has a reference level set to a level V.sub.H corresponding to the intermediate value between demodulated signal voltages corresponding to data "10" and "11", and a reference level set to a level V.sub.L corresponding to the intermediate value between demodulated signal voltages corresponding to data "01" and "00".
FIG. 5 is a graph showing the general S/N characteristics of a conventional FSK demodulation circuit. When the RF signal input level is sufficiently high, the demodulated signal level is constant. When, however, the RF signal input level becomes lower than a certain level (e.g., V.sub.TH), the demodulated signal level decreases with respect to the level V.sub.REF. FIG. 6 is a timing chart for explaining the relationship between an RF signal input level and comparator reference levels in the prior art.
In this conventional FSK demodulation comparator, since the LSB comparator reference levels remain at constant voltages regardless of the magnitude of an RF signal input, the relationship between the LSB comparator reference levels and the demodulated signal amplitude voltage obtained when the RF signal input is large differs from the relationship between the LSB comparator reference levels and the demodulated signal amplitude voltage obtained when the RF signal input is small. This indicates that when the RF signal input becomes lower than a certain level, the LSB comparator cannot perform correct signal determination.